There's also, perhaps, a philosophical element, where we expect 'warfare' to have high stakes, when really it might just be super petty. It's a lot easier for villainous a-holes to justify sending in the military to kill a bunch of civilians if it takes a minimum of 2 weeks for news to reach anywhere else, and if very few people travel between worlds.
Like, think of all the abuses that billionaires get up to in our current world - putting fingers on the scales of elections, buying whole media companies to convince people to be upset about the same stuff they are. They don't hesitate to make life difficult for small fry folks who try to get in the way of their economic goals.
If you're a trillionaire or more, and you have the ability to control multiple planets, and you hear that Nektar III has a really cool island with a great cliff waterfall view next to the ocean, eh, maybe you don't blink at sending a dozen mechs to the planet to kill the local governor and occupy the place so you can take it as your vacation spot.